SS , Thanks for that - I suppose I could have pulled both and checked, but with so many suppliers offering special stubs I assumed that something special was needed.

And Chas , yes it was the early 50's 650 T.birds which managed the
90 MPH/MPG feat.

It was 60 odd years ago when I first read the article on the test, but our local old bike mag is rehashing old "Motorcycle" articles(without any acknowledgement) and I had the chance to read it again in a freshened up form.

I still have a clear recollection of the original article, but I cant remember just what I have done with the new mag------.

SS , Thanks for that - I suppose I could have pulled both and checked, but with so many suppliers offering special stubs I assumed that something special was needed.

And Chas , yes it was the early 50's 650 T.birds which managed the
90 MPH/MPG feat.

It was 60 odd years ago when I first read the article on the test, but our local old bike mag is rehashing old "Motorcycle" articles(without any acknowledgement) and I had the chance to read it again in a freshened up form.

I still have a clear recollection of the original article, but I cant remember just what I have done with the new mag------.

I think the 32mm Bings work better too. I am not much of a fan of the 40mm Bings. I am a big fan of bigger than 32mm carbs, just not Bings.

Whatever the issue is with a square box/40mm Bing combo I don't think it is volume. If 40mm Bings pulled that much more air than 32mm Bings they would run that much stronger with 40mm Bings and they don't. Besides, I know from experience that the stock airbox can pull a lot more air than 32mm Bings on a stock engine for my bike making way more power with a stock square box and 38mm Dellorto's, a sport cam, mild port work, and a sport exhaust.

The standard PD engine has too low a compression ratio to make the most of the 40mm carbs at higher RPM and the standard airbox (tops and all) cause a depression midrange that takes the mixture way too rich effectively killing the power delivery. The 32mm Bings were great and kept the mixture within a very tight band making good power all the way through. I was lucky at the time in having a two GS's, both the same year, one with 32mm carbs and the other with 40mm that I could run back to back to see what the differences were. The 32mm equipped bike was so much better all the way through.

What I do find strange is that BMW chose to produce the R100R and 100GS in Europe with 40mm when the rest of the 1000cc bikes came with 32mm carbs. Makes no sense when the GS especially is not intended as a high speed machine.

The clamshell airbox doesn't cause such a midrange depression in the 40mm carbs mixture as the standard airbox allowing the bike to run cleaner but it cause the mixture to run very weak at the top end. I know Plaka lists a range of jets but I could only buy 160 and 170 in the larger sizes. 160 was too weak and 170 too rich so I ran with the 170 mains along with variations in airbox designs for a while before putting it all back to standard. What the clamshell did do was give a nice progressive power delivery, unfortunatly the GS doesn't have the compression as standard to make the most of it at the top end. At the end of the day there's little point trying to hotrod a standard GS when the chassis, weight, tyres and brakes are the biggest limiting factor. Personally I think the biggest single improvement to the 100GS if you don't want to be messing with high comp pistons, airboxes, exhausts etc is to fit a pair of 32mm Bings.

The standard PD engine has too low a compression ratio to make the most of the 40mm carbs at higher RPM and the standard airbox (tops and all) cause a depression midrange that takes the mixture way too rich effectively killing the power delivery. The 32mm Bings were great and kept the mixture within a very tight band making good power all the way through. I was lucky at the time in having a two GS's, both the same year, one with 32mm carbs and the other with 40mm that I could run back to back to see what the differences were. The 32mm equipped bike was so much better all the way through.

What I do find strange is that BMW chose to produce the R100R and 100GS in Europe with 40mm when the rest of the 1000cc bikes came with 32mm carbs. Makes no sense when the GS especially is not intended as a high speed machine.

The clamshell airbox doesn't cause such a midrange depression in the 40mm carbs mixture as the standard airbox allowing the bike to run cleaner but it cause the mixture to run very weak at the top end. I know Plaka lists a range of jets but I could only buy 160 and 170 in the larger sizes. 160 was too weak and 170 too rich so I ran with the 170 mains along with variations in airbox designs for a while before putting it all back to standard. What the clamshell did do was give a nice progressive power delivery, unfortunatly the GS doesn't have the compression as standard to make the most of it at the top end. At the end of the day there's little point trying to hotrod a standard GS when the chassis, weight, tyres and brakes are the biggest limiting factor. Personally I think the biggest single improvement to the 100GS if you don't want to be messing with high comp pistons, airboxes, exhausts etc is to fit a pair of 32mm Bings.

I agree with you completely about the larger, non bing carbs, and intend to fit MkII Amals on my current project bike.

Which top did you have. It sounds like the one with both horns being the same size? My 8.5:1 Dell/sport cam setup had a big flat spot in the midrange with that top on the box. One large/one small horn fixed that perfectly with almost no penalty on top but the cap with two large horns killed midrange with absolutely stock setups as well. IMO, the GSPD types are too heavy but the G/S's are light. Tires and brakes issues are easily fixed. The single biggest thing I did to my bike which had the same engine as yours excepting lower 8.5:1 CR pistons (I think your came with 9.5:1 pistons?) and coming stock with 32mm Bings was putting a 336 cam in it. Huge step in the right direction! If I was going to go back to 32mm Bings I would rather instead go back to 36/38mm Dell's or Mikuni's. 32mm Bings are great at what they do. My only problem with that is that they don't do much at all to start with!

Bing makes mains for our CV's from 70' to 300's. I keep them at my shop from 120's to 170's. I keep needle jets from 260's to 274's. You can't get 260's and 262's through BMW.

when Truimph did a 24 reliability trial on three stock production SU equipped, unfared, Thunderbird's they managed over 90 MPH and over 90 miles per (imperial) gallon - a bit better than any Bing equipped Airhead is ever going to do.

My 650 Triumph would do 100 imperial mpg with an HS2 SU. But it wasn't a T'Bird, with a 9 stud top end, 9:1 pistons and Bonnie cams....and it wasn't a Triumph either, but a Triton and weighed a lot less than a stock bike. Still, how come my 650 Triumph could do 100mpg, and my 600cc Norton also with an HS2 could do over 90mpg....and yet my 650 BMW of similar hp can't get anywhere near those figures ?

Which top did you have. It sounds like the one with both horns being the same size? My 8.5:1 Dell/sport cam setup had a big flat spot in the midrange with that top on the box. One large/one small horn fixed that perfectly with almost no penalty on top but the cap with two large horns killed midrange with absolutely stock setups as well. IMO, the GSPD types are too heavy but the G/S's are light. Tires and brakes issues are easily fixed. The single biggest thing I did to my bike which had the same engine as yours excepting lower 8.5:1 CR pistons (I think your came with 9.5:1 pistons?) and coming stock with 32mm Bings was putting a 336 cam in it. Huge step in the right direction! If I was going to go back to 32mm Bings I would rather instead go back to 36/38mm Dell's or Mikuni's. 32mm Bings are great at what they do. My only problem with that is that they don't do much at all to start with!

Bing makes mains for our CV's from 70' to 300's. I keep them at my shop from 120's to 170's. I keep needle jets from 260's to 274's. You can't get 260's and 262's through BMW.

I tried no horns, cut down horns and different sized ones but they all still had the vacuum depression mid range.

The standard pd engine has 8.5:1 pistons (mine are standard in the pd).

I have another gs running high comp, moorespeed oversized valves, guides etc but is running 81 heads, which richie isn't keen on due to the port size. It has a standard air box on at the minute and still has the stumble on the midrange where the mixture goes way too rich. You can barely feel it on the road but it shows in the dyno.

It's a pity they didn't plot the air/fuel ratio on there Charles, that dip coincides with mixture dropping as low as 10:1 on my plots. The 32mm carbs kept a nice mixture all the way through so made a lot more power.

It's a pity they didn't plot the air/fuel ratio on there Charles, that dip coincides with mixture dropping as low as 10:1 on my plots. The 32mm carbs kept a nice mixture all the way through so made a lot more power.

Mine is with 38MM dells, i should have asked for the printout to include the mixture.

Dells, a true 40mm exhaust pipe, Jim Cray 40mmsilencers, twin plugged squish checked, ported, Seibenrock 1070 etc etc. nearest I could get to an 80s hot rodded airhead, but using some of the parts than have been developed since.

Dells, a true 40mm exhaust pipe, Jim Cray 40mmsilencers, twin plugged squish checked, ported, Seibenrock 1070 etc etc. nearest I could get to an 80s hot rodded airhead, but using some of the parts than have been developed since.

That's just what I'm putting together at the minute. Right down to the 40mm exhaust system. I'll be going with the clamshell airbox, amal carbs and a 336° Cam.

Not my experience at all. My airbox experiments had the same results regardless of whether or not I had a 32mm Bing or a 38mm Dellorto. A 308 cam or a 336 cam. Stock mufflers or Staintune Sports. Going from one small/one large to two larges horns killed my midrange. No horns killed it worse. A variety of shortened horns and holes in the top like the picture above all did my midrange NO good. It ran by far the best with the one small/one large stock top. Just about perfect in fact. On a dyno and off.

Then everything changed when I raised my CR to a calculated 10.0:1, dual plugged and remapped the ignition curve, went from 42 to 44mm intake valves, and really changed my port shapes. The same above experiments didn't hurt my midrange so much and got me gains on top. A few more tests and I am now running no top at all and my stock velocity stacks are shortened so the the bell mouths are just inside of the box. I have got super strong midrange with a setup that before would have killed it! It all depends.

Not my experience at all. My airbox experiments had the same results regardless of whether or not I had a 32mm Bing or a 38mm Dellorto. A 308 cam or a 336 cam. Stock mufflers or Staintune Sports. Going from one small/one large to two larges horns killed my midrange. No horns killed it worse. A variety of shortened horns and holes in the top like the picture above all did my midrange NO good. It ran by far the best with the one small/one large stock top. Just about perfect in fact. On a dyno and off.

Then everything changed when I raised my CR to a calculated 10.0:1, dual plugged and remapped the ignition curve, went from 42 to 44mm intake valves, and really changed my port shapes. The same above experiments didn't hurt my midrange so much and got me gains on top. A few more tests and I am now running no top at all and my stock velocity stacks are shortened so the the bell mouths are just inside of the box. I have got super strong midrange with a setup that before would have killed it! It all depends.